β-galactosidase (EC 3.2.1.23) is an enzyme which hydrolyzes a β-D-galactoside linkage to release D-galactose, and β-galactosidase isozymes are generally found in a wide range of microorganisms and plants. β-galactosidase is otherwise referred to as lactase. β-galactosidase is also capable of transferring a galactoside linkage, and there are known methods in which this capability is used to produce galacto-oligosaccharides (oligosaccharides having galactose residues).
Various β-galactosidases are known to be produced by a koji mold Aspergillus oryzae, yeasts Kluyveromyces lactis and Kluyveromyces marxinus, a bacterium Bacillus circulans, and others. Among β-galactosidase enzymes produced by these microorganisms, the β-galactosidase derived from Bacillus circulans (see Patent Literature 1 and Non Patent Literature 1) is an enzyme allowing the production of galacto-oligosaccharides from lactose, and is an important enzyme in the industrial production of galacto-oligosaccharides (for example, a preparation of β-galactosidase isozymes is marketed under a trade name of “BIOLACTA”).